Noise when USB is plugged in

kitt350 · 04-26-2009, 05:22 PM

Okay here is what I have. I have an alpine indash dvd player with an aux in that I am running the sound from the computer to there with a 12 foot 3.5mm extension that plugs intoa 3.5mm plug to rca adapter. I have to use the 12 foot since the computer is in the rear of my 89 firebird. If I have this plugged into the computer, either my laptop or desktop that I will be using for my carpc once I get it transferd to its new smaller case, but once I plug in the usb connector to the computer from my dynamix 8" touchscreen I get this audible hum in the speakers. It isn't to noticeable if I keep the volume below 15 but if I go above it you can really start to hear it. It doesn't matter what usb port I use I still get the same sound. Once I unplug the usb from the lcd the sound goes away so something with the screen is causing this kind of feedback. I can hear a popping sound as well when I change video sources on the screen as well. Could this be a grounding problem with the powering of the screen causing this. I search around on here but couldn't really find this kind of problem. All of the usb and audio plugs are all onboard connections. The USB extenstion cable is 10ft long. If I can't find anything to stop this I just might have to not use the touchscreen part and find a touch pad or trackball to use in the car. Thanks for any help anyone can give. If you need any more info please ask and I will give any info needed to try and help solve this problem.

Sponge · 04-27-2009, 01:32 AM

I'm with you... i would suspect the power on the monitor as well. I'm guessing that you are powering your monitor from a 12v constant in the front of the car?

You could try running an extension cable to your car and powering the monitor off of a 120v power adapter and see if its fixes the problem. If thats the case, you could just add a 12v line from your car PSU up to your screen and that could solve it.

bes51659 · 04-27-2009, 06:30 AM

You have a ground loop with the audio and the USB cable.

1) Are the analogue extension and the USB cable drawn the same way from the boot to the LCD panel?
You could have a loop sensitive to magnetic noice that is proportional to area between the two wires. If possible, twin them.

2) You could break up ground loop in audio with standard component, or use a wireless USB hub.

3) You can cut the red and the black wire in the USB cable and source the LCD panel with 5V separately from dash.

Crinos · 04-27-2009, 06:52 AM

Quote: Originally Posted by bes51659

You have a ground loop with the audio and the USB cable.

1) Are the analogue extension and the USB cable drawn the same way from the boot to the LCD panel?
You could have a loop sensitive to magnetic noice that is proportional to area between the two wires. If possible, twin them.

2) You could break up ground loop in audio with standard component, or use a wireless USB hub.

3) You can cut the red and the black wire in the USB cable and source the LCD panel with 5V separately from dash.

Option 3 is quite good, if you do not want to use a wireless usb hub.

Best way to do it is to cut it aprox 1m from the end, and there re-solder white and green back to the usb that where cut off, and then red to a 5v source (while the ignition is on) and black to ground.
5V can be ashieved with a resistor. (usually a 350 ohm ressistor).

NOTE:
Use V = I * R to determine the ressistor. (And keep in mind that there's a different voltage in your car when engine runs, and when it's off, and check up on how many milliampere the USB hub at).

kitt350 · 04-27-2009, 09:31 PM

I think I might have found another solution or found the problem. I put in a new usb 2.0 pci card thinking that might help and it didn't still have the noise. I pulled my radio back and recheck the connections. I have my ps2 sound hooked up to the same rca aux input as the computer using Y splitters so I took out the y splitter and hooked up just the computer and the noise went away, hooked them back up and the noise is back. I guess I just have some bad spitters. I am not using the audio ports on my video switcher so I might just hook them both up to my switcher and the switcher to the aux in on my headunit and see if that keeps the noise away. Hopefully it will so then I won't have to buy any more extra parts and I can get my car put back together. I will let you know how it goes.

CLUCK · 04-27-2009, 11:24 PM

Quote: Originally Posted by bes51659

You have a ground loop with the audio and the USB cable.

1) Are the analogue extension and the USB cable drawn the same way from the boot to the LCD panel?
You could have a loop sensitive to magnetic noice that is proportional to area between the two wires. If possible, twin them.

2) You could break up ground loop in audio with standard component, or use a wireless USB hub.

3) You can cut the red and the black wire in the USB cable and source the LCD panel with 5V separately from dash.

Could you give details of how to achieve the 'twin' ing of the wires, in option 1 ?

Curiosity · 04-28-2009, 02:00 AM

It's actually kind of bad to connect 2 line level outputs direcly to each other. You really need a mixer but they are usually pretty expensive. There are cheaper DIY options though...
http://www.instructables.com/id/SRTI4TPFMEG9TX4/

kitt350 · 04-28-2009, 07:52 AM

When I connected both of them to the input I really didn't think it would be a problem since I would only have one running at a time. Didn't think with one turned on and one turned off I would get the interference that I was getting but I guess I was wrong.

Curiosity · 04-28-2009, 12:14 PM

Oh yes, it can cause all kinds of problems and even damage something by putting an extra load on the output that's on. IIRC, high impedance causes EMI and low causes hum. The hum means something is going to overheat.

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